Fiber optic sensors are considered as valuable replacements to conventional sensors for environments where immunity against electromagnetic field, small size and high operating temperature are required. Numerous types of sensors have been developed over the years including, among others, intensity, interferometric and wavelength based sensors. Intensity based sensors never proved to be reliable, while fiber Bragg gratings require a costly optical demodulator for measuring minute wavelength shift. Fabry-Perot interferometric sensors deliver very good performance, they are flexible as they can be built to measure most physical and chemical parameters, and the sensors themselves can be made at a very low cost. High performance optical demodulators however are still too expensive and bulky too compete with conventional electrical sensors.
In the prior art, high coherence light sources such as lasers are often used for measuring the interferometric differential path length that changes as function of the parameter to be measured. Laser light source are however limited to the measurement of the relative change of interferometric sensors and thus, they are limited to dynamic measurements as taught by U.S. Pat. No. 5,301,001 Murphy et al., Apr. 5, 1994. The precision that can be attained is also strongly dependant on the wavelength stability. Wavelength stability is however costly to achieve.
White light interferometric sensors, using a broadband light source instead of a laser, are best suited for measuring the absolute path length of a sensor interferometer. U.S. Pat. No. 5,392,117 to Belleville et al., and U.S. Pat. No. 5,349,439 to Graindorge et al., teach the use of a Fizeau interferometer for the measurement of the absolute optical path length of a sensing interferometer. U.S. patent application Ser. No. 20030039428 to Okamato et al. shows some methods for the data processing of cross-correlated signals obtained from similar white light interferometric sensors. It is shown that these methods are fairly robust against light source instabilities. But unless the severity of the environment justifies it, these methods are nonetheless too expensive and bulky to compete with conventional techniques. Another drawback comes from the use of low cost silicon CCD array detectors that limit the wavelength of the light source below 1000 nm, thus preventing the use of most telecommunication components in the near infrared, such as LED's and single mode fibers.
U.S. Pat. No. 4,678,904 to Saaski et al. describes relevant teaching on the fabrication of low cost Fabry-Perot sensors using photolithography for the production of the sensors.
The use of silicon micromachining techniques, often referred to as microelectromechanical systems (MEMS), is very well suited for the production of Fabry-Perot interferometric devices. Fabry-Perot MEMS devices can be produced at very low cost while assuring the highest quality. Microelectromechanical tuneable Fabry-Perot filters have thus been proposed in the prior art for making low cost spectrophotometric measurements of interferometric sensors as taught by U.S. Pat. No. 4,859,060 to Katagiri et al. In this case, the Fabry-Perot is of high finesse and is used to scan through selected single wavelength so as to obtain the spectrophotometric content of the light signal. This method is however poorly efficient with a very low throughput that limits speed, resolution, accuracy and long term stability of the measurement. A variety of similar microelectromechanical tuneable filters have been proposed in the prior art for performing similar spectrophotometric light measurements: U.S. Pat. No. 6,424,466 to Flanders; U.S. Pat. No. 6,381,022 to Zavracky; U.S. Pat. No. 6,341,039 to Flanders et al.; U.S. Pat. No. 6,295,130 to Sun et al.; U.S. Pat. No. 5,909,280 to Zavracky; U.S. Pat. No. 5,838,484 to Goossen; U.S. Pat. No. 5,739,945 to Tayebati; U.S. Pat. No. 5,701,193 to Vogel et al.; U.S. Pat. No. 5,561,523 to Blomberg et al.; U.S. Pat. No. 4,825,262 to Mallinson. For the previously proposed electromechanical Fabry-Perot, the finesse is high so that they are being used as spectrophotometers or wavelength filters.
Therefore, there is a need for a high performance low cost micro opto-electromechanical demodulator for efficiently measuring the path length difference of interferometric sensors.